Magnetization transfer (“MT”) is, in general, a macroscopic phenomenon in which two distinct pools of nuclear spins exchange their magnetic polarizations. In the field of magnetic resonance imaging (“MRI”), MT can indicate that one of the two pools can consist of water protons, the amount of which is present in large excess in tissues and organs, and the other can consist of protons associated with macromolecules, between which there can exist either a physical exchange or a magnetization exchange via relaxation pathways such as the Nuclear Overhauser Effect (“NOE”).
The term chemical exchange saturation transfer (“CEST”), can be caused by chemical exchange, especially between water protons and exchangeable solute protons. In tissues and organs, CEST can occur together with MT, which can make the quantitative measure of CEST difficult. In principle, CEST can be distinguished from conventional MT by its frequency selectivity (e.g., CEST can occur in the very narrow range of frequencies, which can be selectively irradiated and compared with MT happening in a wide range of frequencies). It can be difficult, however, to separate the two effects if an asymmetry in MT exists, which can be very common in biological tissues. Previous work described that one could achieve uniform saturation of a strongly-coupled spin system by simultaneously irradiating at two different frequencies that lie within its dipolar coupling-broadened spectral range. If the MT exchange processes occur on a timescale that is slower than the rate of saturation, the two-frequency radio frequency (“RF”) irradiation can uniformly saturate those protons belonging to the macromolecules in tissues and organs. As a result, the magnetization of water protons can diminish through MT exchange processes. Consequently, the MT effect can become independent of the frequency positions of the saturating RF irradiation. Conversely, due to the frequency selectivity of CEST agents, two-frequency RF irradiation does not significantly alter CEST dynamics.
Thus, it may be beneficial to establish a two-pool model for MT based on, for example, Provotorov's theory of saturation and extend the model to describe the dynamics under simultaneous two-frequency irradiation, that can provide an exemplary unencumbered extraction of CEST, and that can overcome at least some of the deficiencies described herein above.